1. Field of Invention
The present invention relates generally to controlling the trajectory of a balloon and more specifically to a control device located remotely from a balloon for providing desired forces for trajectory control.
2. Description of Related Art
Very few devices have been used to control the trajectories of free balloons, such as balloons carrying scientific atmospheric sensing instruments. Propeller-driven airships can control their trajectories, for example, through modulation of the speed and the pitch of a propeller. However, the attainable altitudes and payload masses for airships are quite restricted in comparison to those of free balloons. Free balloons carrying science instruments typically drift freely in the prevailing wind at a desired operating altitude. In many cases, launch of such balloons must be delayed until forecast winds are projected to carry the balloon system into a region of interest or away from a forbidden zone. Frequently, such balloon flights must be prematurely terminated to avoid flying over specified areas, to ensure that the payload can descend onto an appropriate landing site, or to avoid endangering densely populated regions. The ability to provide even a small amount of trajectory control could eliminate these reasons for termination.
Previous approaches considered to control the trajectory of free balloons have included propellers, altitude control to select different wind directions, and drag chutes on long tethers.
Propellers require substantial power to drag a balloon through the atmosphere. The air has very low density at the high altitudes typically required of scientific balloons. At these high altitudes, propellers must be quite large in order to generate substantial lift. Also, significant amounts of power are typically unavailable for balloon systems due to the inherent need to keep weight to a minimum. If the power is generated using solar cells, then nighttime operation is not possible without very heavy batteries. If combustion provides the propulsive power, then the duration is limited by the weight of portable fuel. These requirements for propulsive power are at odds with the need to keep the weight low.
Several studies have been performed of concepts to propel lighter-than-air (LTA) vehicles. Naturally-shaped balloons driven by propellers suspended on relatively short tethers are discussed in xe2x80x9cA Comparison of Several Very High Altitude Station Keeping Balloon Concepts,xe2x80x9d by J. J. Vorachek, presented at 6th AFCRL Symposium, 1970, and xe2x80x9cAdvanced Balloon Systems as Photographic Platforms,xe2x80x9d by R. R. Ross, presented at Earth Observations From Balloons, a Symposium, 1969. Both references discuss studies wherein naturally shaped balloons with a propeller and a power plant, both suspended on a tether, were tested in flight. According to these references, the operation of such devices would be limited to a couple of days due to the large propulsive energy required. These references also discuss the difficulties associated with operating engines at high altitude. Air breathing engines require several stages of supercharging to increase the density to the point that it will burn efficiently with fuel. In addition, both combustion engines and electric engines suffer from the difficulty of rejecting the substantial waste heat to prevent overheating in the low density atmosphere.
Another propeller driven LTA vehicle was designed, as described in xe2x80x9cPOBAL-S, The Analysis and Design of a High Altitude Airship,xe2x80x9d prepared for Air Force Cambridge Research Laboratories by Jack Beemer, et al., of Raven Industries in 1975. This document describes a propeller-driven airship designed to operate at an altitude of 21 km for a period of about a week.
Both the propeller-driven balloon and the propeller-driven airship described above were designed to maintain the position of an LTA vehicle above a specific point on the ground. Such operation requires the LTA vehicle to fly at a relative speed equal to the wind speed at the operating altitude. Since winds can have speeds in the range 15-50 m/s (50-150 ft/s), this leads to significant power requirements.
An alternative approach is to control the altitude of an LTA vehicle to select an altitude at which the wind is moving in a favorable direction (or at least close to a desired direction). This is the main trajectory control technique used by sport balloonists with either hot-air balloons or helium balloons. Selecting altitudes at which the balloon will float in order to select different drift directions also has many drawbacks. First, some means of controlling altitude must be provided. Operators of hot air sport balloons can raise or lower the temperature of the lifting gas to adjust altitude, while operators of helium balloons tend to alternate between dropping ballast weight and venting lifting gas. This use of consumables ultimately limits the duration of the mission. Furthermore, carrying the ballast reduces the weight available for the payload. Another drawback is that many balloon-borne science instruments, especially those used in astronomy and astrophysics experiments, need to be above most of the atmosphere (99%) and cannot acquire high quality data at lower altitudes. Furthermore, good knowledge of the wind is needed at different altitudes in order to select an appropriate altitude. Such detailed knowledge is usually unavailable during the flight. Thus, sport balloon flying often involves significant trial-and-error in seeking favorable altitudes.
One approach for the control of altitude without the use of ballast involved one balloon filled with helium and a second bag filled with a much denser refrigerant. At low altitudes, the refrigerant was a gas. At high altitude, the refrigerant condensed into a liquid. Thus, above a certain altitude, since the displaced volume of air decreased by the volume of gas that condensed, the overall buoyancy of the system decreased. As the system descended into warmer air at a lower altitude, the liquid vaporized again, thus expanding to displace a large volume of air. At this point, the buoyancy exceeded the weight and the system ascended again. The system naturally cycled over a large range of altitudes without discharging helium or ballast. This approach is described in xe2x80x9cBalloon Altitude Control Experiment (ALICE),xe2x80x9d by K. T. Nock, K. M. Aaron, et al. 11th AIAA Lighter-than-Air Systems Technology Conference, 1995. By trapping the liquid refrigerant in a pressure vessel and releasing it back into the sealed bag, it would be possible to provide some control over the altitude. The time scale involved for each altitude cycle was a few hours. This altitude control scheme does not work in the stratosphere; it requires the particular variation of temperature and pressure in the troposphere.
A drag device, such as a parachute, can be deployed a significant altitude below a balloon where the winds will usually be blowing in a different direction. Such an approach is described in Raytheon report R69-4041A, xe2x80x9cUnique Approach to Balloon Station Keeping,xe2x80x9d by E. R. Bourke II, 1969. This approach can be used to generate a force that will cause the balloon to move relative to the surrounding air. However, the direction of the force is restricted essentially to the direction the wind is blowing at the altitude of the parachute. It is possible to use a winch to raise or lower the parachute to altitudes with different wind directions, but this may require a significant amount of time for changing the direction of the force. Also, a significant amount of power may be required to raise such a device in the presence of both gravity and aerodynamic drag. In addition, good knowledge of the wind distribution with altitude is required.
Accordingly, it is an object of this invention to provide an efficient force-generating device to control the trajectory of a balloon. The force-generating device should preferably be oriented so that the lift force is predominantly horizontal and transverse to the motion of the balloon.
It is a further object of the current invention to provide a balloon control device that passively exploits natural wind conditions, permits the balloon to remain at a fixed altitude and induces air flow past the balloon to sweep away contaminants.
It is a further object of the current invention to provide a device which requires very little power and can be operated efficiently at night.
The above and related objects of the present invention are realized by a system that includes a lift-generating device arranged to provide most of its lift in a horizontal direction, suspended from a balloon or other lighter-than-air system on a tether. The system may also include a flap or rudder for the purpose of changing the lift generated by the lift-generating device. The lift-generating device and flap (or rudder) may be mounted on a frame, such as a boom, or else integrally joined.
According to one aspect of the invention, the lift-generating device includes a sail. In another aspect, the lift-generating device includes a wing. In a third aspect, the lift-generating device includes a portion of a whirligig arrangement.
The flap may operate to change the lift generated by the lift-generating device by means of a change in orientation or motion of the lift-generating device. The tether is preferably sufficiently long so as to take advantage of natural wind differences with altitude.
The inventive device disclosed uses very little power to operate, can operate at night, can be made of very lightweight materials, does not require detailed knowledge of the wind field, allows the balloon to remain at a fixed altitude, generates control forces having a greater range of magnitude and direction as compared with a comparably sized drag device, can change the direction of the control force fairly rapidly, and operates at an altitude lower than the balloon where the air density is greater so that the device can be relatively small compared with the balloon.
Another advantage relates to the support of scientific instruments. Some very sensitive science instruments measure trace gases in the atmosphere at very low concentrations of a few parts per billion. Contaminants from the balloon itself, such as the helium lifting gas, or volatiles from the envelope material, can interfere with these sensitive measurements. A typical balloon drifts along with the local air mass and these contaminants accumulate in the vicinity of the balloon and gondola. Even a small relative airflow, such as would be caused by operation of the trajectory control device, will sweep away these contaminants and provide a fresh flow of air samples to the science instruments.
Significant variations with altitude exist for wind speed and direction. By operating the force-generating device well below the balloon (perhaps several kilometers below), a significant wind difference between the balloon and the force generating device is essentially guaranteed. The direction of the wind is not overly important since the magnitude and direction of the lift force can be varied over a substantial range by controlling the angle of attack of the wing, much like the ability of sailboats to travel in many different directions in the same wind.
Further objects and advantages of the current invention will become apparent from a consideration of the drawings and detailed descriptions.